The communication between neurons and microglia is bidirectional. Microglia modulate neurotransmission, facilitate synapse formation and dissolution, and provide neuronal protection, but cellular/molecular mechanisms are incompletely understood. Much of the premise for interactions between dopamine neurons and microglia is supported by presence of dopamine receptors on microglial cells allowing them to respond to neuronal signals. The idea is that dopamine receptor stimulation on microglial cells alters microglial function, which in turn could then (reciprocally) affect dopamine neurotransmission. Here we will use patch clamp electrophysiology, two-photon imaging, biochemical and histological approaches to determine whether and how depleting microglia affects dopamine neurotransmission and whether HIV-1 Tat, a protein produced in microglial cells following HIV-1 infection, disrupts dopamine neurotransmission by altering microglial/DA neurons interactions (Aim 1). We will examine how microglial activity is affected by dopaminergic signaling in the presence or absence of HIV- 1 Tat, and conversely how microglial products may modulate dopamine neurotransmission (Aim 2). Finally, since there is a high comorbidity between HIV-1 infection and drug abuse, and since both methamphetamine and HIV-1 Tat alter dopamine neurotransmission and affect the immune system, in Aim 3 we will determine how the combined exposure to HIV-1 Tat and methamphetamine influences these processes. The proposed work will address two significant knowledge gaps: 1) reveal the cellular/molecular mechanisms underlying bidirectional communication between dopamine neurons and microglia, and 2) determine how HIV-1 Tat modulation of this bidirectional communication reduces dopamine neurotransmission.